1. Field of the Invention
The present invention relates to a half-bridge CCFL driving apparatus. In particular, this invention relates to a driving apparatus for driving CCFLs.
2. Description of the Related Art
The power supply of the backlight of TFT panels usually adopts an inverter circuit to convert the energy and drive the CCFL. The inverter circuit of the prior art can be divided into a half-bridge inverter circuit, a full-bridge inverter circuit and a push-pull inverter circuit. The inverter circuit converts DC into AC.
Please refer to FIG. 1, which shows a schematic diagram of the circuit of a power supply for the backlight of TFT panel. In the circuit, the backlight of the LCD panel is composed of an AC-DC converter 10 and a DC-AC inverter 20 that are connected to each other in series. The AC-DC converter 10 receives an AC power. The AC power flows through the bridge rectifier BD1 and a high-voltage capacitor C1 to form a high-voltage DC voltage on the high-voltage capacitor C1. The PWM controller 12 located on the primary side of the transformer T1 controls the switching element SW1 to generate a high speed switching operation. Therefore, the high-voltage DC voltage is transmitted to the transformer T1. The transformer T1 transfers the power to the secondary winding of the transformer T1. Then, the voltage is filtered via a regulating diode D1 and a filtering capacitor C4 to form a DC voltage VDD on the filtering capacitor C4 that is between 12V to 24V. The DC voltage is supplied to the DC-AC inverter 20. The capacitors C5 and C6 in the DC-AC inverter 20 average the voltage on the filtering capacitor C4. Therefore, the voltage for each of the capacitors C5 and C6 is a half of the voltage on the filtering capacitor C4.
When the PWM controller 14 switches the switching element SW2 on, the voltage on the capacitor C5 provides power from the positive port of the capacitor C5. Then, the power flows through the switching element SW2, the capacitor C7 and the primary winding of the transformer T2 and comes back to the negative port of the capacitor C5. When the PWM controller 14 switches the switching element SW3 on, the voltage on the capacitor C6 provides power from the positive port of the capacitor C6. Then, the power flows through the primary winding of the transformer T2, the capacitor C7 and the switching element SW3 and comes back to the negative port of the capacitor C6. The PWM controller 14 generates a pulse control signal with 180 degrees phase to execute a high-speed switching operation to the switching elements SW2 and SW3. The transformer T2 transfers the power to the secondary winding of the transformer T2 and utilizes the serial leakage inductance produced at the secondary winding of the transformer T2 to work with the resonant capacitors C9, C10. Therefore, a continuous sine-wave voltage is generated and supplied to the CCFL.
Please refer to FIG. 1 again. If the efficiency of the AC-DC converter 10 and the DC-AC inverter 20 both are 85%, then the total efficiency of the serial circuit is 72.2%. The total efficiency is decreased.